Tandem Repeats Contribute to Coding Sequence Variation in Bumblebees (Hymenoptera: Apidae)

Tandem repeats (TRs) are highly dynamic regions of the genome. Mutations at these loci represent a significant source of genetic variation and can facilitate rapid adaptation. Bumblebees are important pollinating insects occupying a wide range of habitats. However, to date, molecular mechanisms underlying the potential adaptation of bumblebees to diverse habitats are largely unknown. In the present study, we investigate how TRs contribute to genetic variation in bumblebees, thus potentially facilitating adaptation. We identified 26,595 TRs from the assembled 18 chromosome sequences of the buff-tailed bumblebee (Bombus terrestris), 66.7% of which reside in genic regions. We also compared TRs found in B. terrestris with those present in the assembled genome sequence of a congener, B. impatiens. We found that a total of 1,137 TRs were variable in length between the two sequenced bumblebee species, and further analysis reveals that 101 of them are located within coding regions. These 101 TRs are responsible for coding sequence variation and correspond to protein sequence length variation between the two bumblebee species. The variability of identified TRs in coding regions between bumblebees was confirmed by PCR amplification of a subset of loci. Functional classification of bumblebee genes where coding sequences include variable-length TRs suggests that a majority of genes (87%) that could be assigned to a protein class are related to transcriptional regulation. Our results show that TRs contribute to coding sequence variation in bumblebees, and thus may facilitate the adaptation of bumblebees through diversifying proteins involved in controlling gene expression.